President Donald Trump is making good on a campaign promise to reverse a key environmental achievement of former President Barack Obama.

Trump on Tuesday signed an executive order targeting the controversial Waters of the United States rule, also known as the Clean Water Rule, as he advances his assault on the Environmental Protection Agency and environmental protections. Another executive order targeting Obamas Clean Power Plan could be signed as soon as this week, according to previous reports .

Tuesdays order instructs the EPA and Army Corps of Engineers to review and reconsider the rule, according to The Washington Post and other media outlets. Final language has not yet been released.

The rule, implemented in 2015, was intended to clarify which of the nations waterways should be protected under the federal Clean Water Act. It includes streams, wetlands and other smaller waterways that collectively provide drinking water for an estimated 117 million people one in three Americans, according to the EPA.

The rule has been tied up in courts, however, with some 31 states , businesses and agricultural groups characterizing it as federal government overreach that would cripple industry.

A vocal critic of the rule has been EPA Administrator Scott Pruitt , who as Oklahoma attorney general sued the EPA over the rule, claiming it was unconstitutional and burdensome to farmers, developers and property owners. Pruitt reiterated his opposition to the rule, during an address at the Conservative Political Action Conference over the weekend, citing it as an example of overregulation.

Despite the Trump administrations strong opposition to the rule, reversing it is no easy matter, triggering a new, potentially lengthy, rule-making process.

Its going to be hard to walk away from the huge scientific record the rule is based on, Jon Devine, senior attorney at the Natural Resources Defense Council, told HuffPost. If they try to run away from the legal test, that would be not only bad policy, but terrible law.